This invention relates generally to the field of centrifugal compressors, and more particularly to a method for limiting split ring diffuser travel.
One of the major problems arising in the use of centrifugal vapor compressors for applications where the compressor load varies over a wide range is flow stabilization through the compressor. The compressor inlet, impeller and diffuser passages must be sized to provide for the maximum volumetric flow rate desired. When there is a low volumetric flow rate through such a compressor, the flow becomes unstable. As the volumetric flow rate is decreased from a stable range, a range of slightly unstable flow is entered. In this range, there appears to be a partial reversal of flow in the diffuser passage, creating noises and lowering the compressor efficiency. Below this range, the compressor enters what is known as surge, wherein there are periodic complete flow reversals in the diffuser passage, destroying the By efficiency of the machine and endangering the integrity of the machine elements. Since a wide range of volumetric flow rates is desirable in many compressor applications, numerous modifications have been suggested to improve flow stability at low volumetric flow rates.
Many schemes have been devised to maintain high machine efficiencies over a wide operation range. In U.S. Pat. No. 4,070,123, the entire impeller wheel configuration is varied in response to load changes in an effort to match the machine performance with the changing load demands. Adjustable diffuser flow restrictors are also described in U.S. Pat. No. 3,362,625 which serve to regulate the flow within the diffuser in an effort to improve stability at low volumetric flow rates.
A common technique for maintaining high operating efficiency over a wide flow range in a centrifugal machine is through use of the variable width diffuser in conjunction with fixed diffuser guide vanes.
U.S. Pat. Nos. 2,996,996 and 4,378,194 describe variable width vaned diffusers wherein the diffuser vanes are securely affixed, as by bolting to one of the diffuser walls. The vanes are adapted to pass through openings formed in the other wall thus permitting the geometry of the diffuser to be changed in response to changing load conditions.
Fixedly mounting the diffuser blades to one of the diffuser walls presents a number of problems particularly in regard to the manufacture, maintenance and operation of the machine. Little space is afforded for securing the vanes in the assembly. Any misalignment of the vanes will cause the vane to bind or rub against the opposite wall as it is repositioned. Similarly, if one or more vanes in the series has to be replaced in the assembly, the entire machine generally has to be taken apart in order to effect the replacement.
The efficiency of a compressor could be greatly enhanced by varying the outlet geometry of the diffuser. A variable geometry pipe diffuser is disclosed in U.S. Pat. No. 5,807,071. A variable geometry pipe diffuser (which may also be termed a split-ring pipe diffuser) splits the diffuser into a first, inner ring and a second outer ring. The inner and outer rings have complementary inlet flow channel sections formed therein. That is, each inlet flow channel section of the inner ring has a complementary inlet flow channel section formed in the outer ring. The inner ring and outer ring are rotatable respective one another. The rings are rotated to improve efficiency for varying pressure levels between a fully open position and a partially closed position. In the partially closed position the misalignment of the exit pipes of the diffuser causes an increase in noise. Rotation of the rings past an optimum design point results in excessive noise and efficiency degradation.
The geometrical tolerances within a centrifugal compressor are small. At the same time the loads within the compressor are large and dynamic in nature. In a split ring pipe diffuser the problem of maintaining tolerances in the face of the dynamic loading becomes quite onerous. There are both axial (thrust) loads and circumferential loads on the ring pair that need to be managed. The diffuser rings must be able to rotate relative to one another and at the same time tight control over their relative position must be maintained in order to ensure proper alignment of the flow channels and the ultimate efficiency of the compressor. The cost of maintaining the necessary tolerances in a split ring diffuser is generally very high.
Another problem with split ring diffusers is premature part wear. Lubricants are generally not used within the gas flow regions of centrifugal compressors to preclude contamination of the gases. The dynamic loads imposed upon the split ring diffuser by the gas flow exiting the impeller cause wear in the components of the diffuser to be accelerated by the absence of lubricating oil.
The drive system for accurately positioning the rings relative to one another must, among other things, be rigid to avoid any fretting of components. Because of circumferential loading on the rings there is a propensity for the inner ring to oscillate relative to the outer ring which could cause compressor instability, part wear and could adversely affect efficiency. This causes several problems that need to be overcome. A drive system is needed that is capable of preventing the relative movement between the inner and outer rings. A bearing concept is also needed which would allow for the relative rotation of the two rings and also be capable of withstanding the circumferential and thrust loads while maintaining tight geometric tolerances between the rings. There is also a need to provide a positioning system that includes positive minimum and maximum stops to avoid unnecessary noise and efficiency degradation as well as simple field retrofit. In addition, there is a need for the drive and bearing systems have a long operating life and be easy to install and adjust properly. U.S. Pat. Nos. 5,895,204; 5,988,977; and 6,015,259 address these concerns.
Briefly stated, a diffuser for a centrifugal compressor includes an inner ring and an outer ring. An actuator drives a pinion gear which interfaces with a rack gear which is mounted to the inner ring. The actuator is operable to position the inner ring between a fully open position and a partially closed position with respect to the outer ring. In the fully open position, the rack gear is offset from an outer ring stop a distance equivalent to five degrees of actuator movement. The control logic of the actuator is modified so that the open position of the actuator moves the pinion gear so that the rack gear stops at or near the offset distance, leaving five degrees as the margin for error. Any overtravel by the actuator does not cause the rack gear to impact the outer ring stop, thus avoiding possible stall of the actuator.
According to an embodiment of the invention, in a centrifugal compressor having a casing and an impeller rotatably mounted therein for bringing a working fluid from an inlet to the entrance of an annular radially disposed split ring diffuser, the diffuser including an inner ring, the inner ring having a plurality of first channel sections formed therein, an outer ring, the outer ring having a plurality of second channel sections formed therein, each second channel section having a complementary first channel section; the compressor including a drive positioning mechanism for rotating the inner ring circumferentially within the outer ring between a first, fully open position wherein the complementary first and second channel sections are aligned to allow a maximum flow of fluid through the complementary channel sections, and a second, partially closed position, wherein the first and second complementary flow guide channels are misaligned to restrict flow of fluid through the complementary channel sections, wherein the drive positioning mechanism includes an actuator; a pinion axle rotationally driven by the actuator at a first end of the pinion axle; a pinion gear mounted to a second end of the pinion axle; a rack gear fixedly mounted to the inner ring extending radially outwardly from the inner ring and adapted to engage in meshing arrangement with the pinion gear; and first and second limit stops in the actuator for limiting travel of the inner ring between the first position and the second position; the method includes the steps of (a) mounting the rack gear a predetermined distance from an outer ring stop such that the first channel sections and the second channel sections are aligned in the open position; (b) moving the rack gear against the outer ring stop; (c) installing the actuator and the pinion gear so that a full open position of the actuator, represented by a first amount of control current, positions the rack gear against the outer ring stop; (d) translating the predetermined distance to degrees rotation of the actuator; (e) translating the degrees rotation of the actuator to a second amount of control current; and (f) modifying control logic which controls the actuator so that moving the actuator to bring the inner ring into the first position is accomplished by applying a third amount of control current to the actuator, wherein the third amount of control current is equal to the first amount of control current added to the second amount of control current.